process a great tragedy that unfolded here this morning. A little while ago, I was just down the street where a thousand people gathered inside and outside saint rose catholic church here in newtown, connecticut, to mourn the loss of 26 people, including 20 school children almost between the ages of 5 and 10 years old, shot and killed in their elementary school. Classrooms this morning, schools in this town are part of the draw for people who live here, with some of the highest test scores in the state. Move to for the sake of their children's school. Dan harris has been at the scene since early today. Dan? Reporter: Cynthia, good evening. It started just a little behind this volunteer fire department, down the road at a suburban tary school, which tonight is in the history books for the worst of all possible reasons. And this town, which one parent today described as the safest place in america, is in total shock. 90 minutes into the school day on a bright winter morning here, police got a call from the sandy hook elementary school. The caller indicating she thinks there's a shooter somewhere in the building. Reporter: At around 9:30, the shooter, 20-year-old adam lanza, a former student in the local schools, shown here as a teenager, had barged in. According to police, he was heavily arm, coated in body armor and ready to kill. We heard this racket at our classroom, and we were, like, all scared. Then we heard them say go in your cubbies. Reporter: There were more than 600 children from kindergarten through fourth grade inside. I was in the art room. We were doing art. We heard . It was really scary. The teachsaid sit down inside the corner and be quiet. Reporter: Calls went out to police even as shots were still being fired, and within five to ten minutes, tactical teams with long guns drawn swarmed the school, a stark contrast against the backdrop of this bucolic woodsy neighborhood. I have reports of two shooters running past the jim, which would be in the rear. I need units in the school. I got bodies here. Reporter: Petrified students and teachers looked for safe places to hide. One teacher locking her children in a bathroom as police combed the building looking for the shooter. Police immediately upon arrival began a complete search of the building, including checking every door, every crack, every crevice of that school. We heard screaming, and then the police came in. It's like, is he in here? And then he ran out. Reporter:8-year-old alexis was in her third grade classroom at the time. We heard an balance and the police officer come, and everybody was a little scared, crying. I felt actually a little sick. I felt likgoing to throw up. Reporter: Were kids crying and screaming? Yeah. Kids were crying. Not really screaming, but they were all hugging together and they felt so sick. Reporter: Teachers led the students single file from the schoolhouse to a nearby fire station. We got in line and had to close our eyes. We put our hands on other people's shoulders, and then our teacher held the first person's hand and then she led us out. Reporter: Parents were alerted by text message and a frantic parade of them arrived at the fire station hoping to find their children alive. This is heartbreaking. It's heartbreaking. When I heard, I started crying. Reporter: Alexis parents were among the lucky. Her mom says when she arrived at the fire house, alexis was already with her father. He had her in his arms and crying. So she just goes momma, I'm okay, I'm here. RepHE RAMPAGE AND THE Evacuation all happened in a flash, but we wouldn't know the full scope of the horror for hours. The shoot ser deceased. The scene is secure. Reporter: By 10:30, president obama was being briefed on the situation and schools throughout the area were on lockdown. Police were surrounding a second crime scene, the sheert's mother's home, nearby. At around noon, the horrifying numbers were announced. Abc news learned that lanza had killed 27 people, most of them children. At the local hospital, they were braced for an onslaught but they only got three patients, two of whom died. A number of us imagined what it must have felt like on 9/11 when there were people bracing for scores of injured and they didn't arrive. Reporter: At the fire house were students were skill gathered waiting for their parents, we met another mother whose children go to a nearby school. She told us she was here to support her friends and to put off picking up her own children because she didn't want to tell them about this massacre. Two hours of their life they aren't going to forget. I know there's not a parent in america who doesn't feel the same overwhelming grief that i do. Reporter: Shortly after 3:00 in the afternoon, the president addressed the nation. The majority of those who died today were children. the ages of 5 and 10 years old. Reporter: Not long after that, police announced that lanza had shot his mother, who was believed to have worked at the school at some point, at her home before he then went and stormed the school. We need to establish identity. We need to document the entire scene and simply say that we need to answer every single question surrounding exactly how and why this incident occurred. Reporter: Tonight at the capitol building in washington, flags lowered to half staff. While back in newtown, true to the grim and all too familiar script of these mass shootings, a candle light vigil. All day long as we watched the drama unfold here, adding to the shock in this community, the fact that this is such an unlikely venue for violence. A small sleepy town with a population of fewer than 30,000, the kind of bucolic new england place where people move precisely for the schools and the safety. It's like, should I be pinched to see if it really is real? It's just heart wrenching. Reporter: Feels like a dream, and a bad dream. It's a nightmare. It feels like a nightmare. Reporter: And for those lucky enough to be reunited with their children tonight, a wake-up call. And you count your blessings every day for what you have. Reporter: Alexis, I'm really glad you're okay. I'm really glad you're okay. You're pretty tough. You're pretty tough. She is a great kid. And she's had a very tough day. And tonight, abc news has learned that the fbi is interviewing friends and family of the gunman, not only here in connecticut, but also in new jersey and in massachusetts as they try to go about answering the big burning question -- why? What was his motive? Cynthia?

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